Surface acoustic wave devices are small-sized and stable to temperature and to variations with time, can be adapted to give the desired filter characteristics by varying the configuration of comblike electrodes and therefore find wide application, for example, to IF filters for television receivers, IF filters for satellite broadcasting, RF converter oscillators for VTRs, etc.
In recent years, especially, great efforts are directed to the development of high frequency surface acoustic wave devices for use in pocket pagers, mobile radio telephones and the like. Attention has been focused on surface acoustic wave devices of this type which include a single crystal aluminum nitride film or a C-axis-oriented aluminum nitride film as a piezoelectric film assuring a high wave propagation velocity.
Surface acoustic wave devices incorporating a single crystal aluminum nitride film are disclosed in IEEE Transaction on Sonics and Ultrasonics, Vol. SU-32, No. 5, September 1985, "Zero-Temperature-Coefficient SAW Devices on AlN Epitaxial Films." Further surface acoustic wave devices including a c-axis-oriented aluminum nitride film are disclosed in Applied Physics Letter, Vol. 36, No. 8, pp. 643-645, 15 Apr. 1980, "Low-temperature growth of piezoelectric AlN film by rf reactive planar magnetron sputtering."
It is desired that surface acoustic wave devices be as great as possible in effective surface acoustic wave coupling coefficient K.sup.2 which represents the efficiency of conversion of electrical energy to surface acoustic wave energy. For example, it has been proposed to give an improved coupling coefficient to surface acoustic wave devices employing a single crystal lithium tetraborate substrate by suitably selecting the direction of propagation of the wave (Unexamined Japanese Patent Publication SHO 59-4309).
However, full research has yet to be made on improvements in the coupling coefficient of surface acoustic wave devices employing an aluminum nitride (AlN) film. For example, the device disclosed in the foregoing IEEE Transaction, which comprises a single crystal film of (1120)[0001] AlN formed on a substrate of (0112)[0111] Al.sub.2 O.sub.3 is as low as 0.8% in coupling coefficient K.sup.2.
The device having a c-axis-oriented aluminum nitride film and disclosed in the above-mentioned Applied Physicas Letter has a very low coupling coefficient K.sup.2 of 0.12%.